The invention relates to a screen for laser rear projection which, selectively for one or several laser wavelengths, forward-scatters the incident, narrow-band laser radiation in a previously determined solid angle but simultaneously highly absorbs the stray broad-band ambient light.
The necessity of visualizing information is currently increasing enormously. For display technology, therefore, this opens up a market with many segments, high sales, and high growth rates. In the functional chain composed of image recording, transmission, and processing of information, considerable progress has been made in recent years. However, the quality of current display methods is no longer adequate for this high state of development. The displays in use today such as cathode ray tubes and liquid crystal displays are limited in their potential for improvement.
Laser display technology, in other words the sequential buildup of images by laser beam deflection without screen afterglow, offers an inherent potential for producing high-quality images. The image information is modulated serially with electro-optical or acousto-optical modulators suitable to the laser beam. The beam is deflected by mechanical mirror scanners, similar to conventional television tubes, linewise over the image surface. Laser sources are being developed by several projector manufacturers as a replacement for thermal radiators such as halogen lamps and discharge lamps in conventional light valve projectors with liquid crystal or micro-mirror matrices.
Basically, thermal radiators are limited in their brightness by their internal operating temperature (black-body radiation). The spectral distribution of the basic colors red, green, and blue (RGB) can be optimized only by appropriate filtration and intensity matching. In addition, a lamp of this kind radiates uniformly in all directions. Despite a rear mirror, only a portion of the total radiation is available for radiation in a particular projection direction.
On the other hand, with a laser any beam power can theoretically be produced and is available in a closely bundled beam with almost zero losses. In addition, as a result of the narrow spectral bandwidth, its power can be converted completely for the required basic color. In recent years, semiconductor lasers and solid-state lasers have been developed with a much higher efficiency (lumens per watt) and a much longer lifetime (more than 50,000 hours) than with conventional lamps.
The theoretical suitability of laser beams for display technology was recognized early on. Hitachi demonstrated color television large projection in 1970. General Electric, Texas Instruments, General Telephone and Electronic Labs, and others demonstrated similar systems in the 1970s. At that time, however, the available lasers (gas lasers) had a lower efficiency, were bulky, and were unsuited for economical mass production.
As a result of the clear technological advances in the development of laser sources, image modulators, and image scanners, completely new technical and economic opportunities are available today for laser display technology:
the possibility of creating the suitable basic colors, red, green, and blue, with high efficiency; PA1 a very high quality of projection as regards brightness, contrast, and resolution, and PA1 manufacturing economics by miniaturization and functional integration.
The laser is a coherent, monochromatically bundled light source. Any required beam density can theoretically be created. In addition, as a result of the narrow bandwidth of its power, the latter can be completely converted for the necessary basic color. With sequential pointwise image creation on the screen, the image is sharp at any distance and can be projected on sloping and curved surfaces as well.
With all the projectors, an image can basically be generated in two ways on the screen: by front projection and by rear projection. In the former case, the image is cast on the surface of the screen on which it is viewed. In this case, the screen must diffusely backscatter the incident light as much as possible. In the latter case, the image is projected on the opposite side of the screen (from the rear). In this case, the screen should allow the light to pass through as completely as possible, but at the same time must forward-scatter over a greater angle. The invention relates exclusively to this second method, rear projection.
Screens for rear projection with conventional image projectors (beamers) are commercially available in different sizes with different screen materials. The screen scatters the light beams directed at every image point on the rear by scattering at the surface or also by multiple scattering in the interior of a thin layer of the material. In this way, the image point on the viewing side is radiated outward diffusely from the screen as an expanded beam bundle. This phenomenon is well known for example from matte disks that are also used in the viewfinders of cameras.
Since the scattering of the light at the surface or within the image material results not only in the light being conducted farther in the forward direction but also to a certain extent in a backscattering of light from the projection surface, this light transmission is always subject to losses. A second disadvantage of these screens is that bright light from the viewing area also enters the screen and is not only conducted through the screen but is also backscattered to a certain degree. As a result, the screen always appears bright in an illuminated room, depending on the ratio of the forward-scattering intensity to the backscattering intensity, in various levels of gray.
In order to compensate for these losses and to reduce the brightness of the screen, so-called lenticular images or lenticular walls are frequently employed. With a fine lenticular pattern on the light exit surface to the viewer, the angle of forward scattering is narrowed. As a result, the image can be seen only in a narrow range of angles around the normal and brightened. At the same time, the recording angle of the projection wall for lateral stray light, for example originating from the lateral illumination of the room, which is directed toward the viewer, is reduced. All in all, these screens, despite illumination from the viewing area, appear darker and at the same time produce a brighter image. Of course, large screens of the latter type are very expensive to manufacture. However since they offer only limited improvements, projection can take place with the desired quality only in half-darkened rooms but not in bright rooms or in daylight.
In the case of front projection onto screens, the backscattering coating of the screen is designed so that the light from the projector is backscattered by this screen in a limited image angle, with the same advantage as in improved rear projection screens.
One major advantage of using lasers for projection is that, as a result of the resultant higher possible beam density by comparison with other light sources, the brightness of the image on the screen can be definitely increased. However, this advantage can be utilized only to a limited extent to increase image quality because in this case the same is true as in television technology: only by creating the image on a black screen can the image contrast be inherently created by the device, as well as the color saturation be transmitted undistorted to the viewer. This is because the minimum contrast perceived by the eye is proportional to the basic brightness of the image (Weber-Fechner Law), in other words the higher the brightness, the lower the perceptible contrast.
In color reproduction in bright rooms, the usually gray or white basic brightness of the screen is added to the color valence produced by the projector, and so the saturation of the color hues of the entire image changes at the same time that the contrast decreases.
This problem of high-quality image reproduction of contrast and color in bright rooms occurs both in front projection and in rear projection. The invention relates exclusively to the second method of rear projection, in other words the reproduction of the image conducted through screen and the design and manufacture of such screen for laser projection.
The measure already explained for increasing image brightness on the screen by narrowing the radiation angle with simultaneous suppression of ambient light, that makes the screen appear gray, also improves contrast and color reproduction. The darker the screen, the better the reproduction.
In color television technology with phosphorescent cathode ray tubes, this has been achieved by using gray glass in the image tube and with the aid of so-called "black matrix" screens. The glass layer of the gray glass damps the light passing through it by one-half. The maximum image brightness is reduced accordingly. Ambient light that strikes the white-reflecting phosphor layer behind the gray glass and thus brightens the image field must pass through the glass twice and is therefore reduced to 25 percent. As a result, a doubling of the contrast advantageously occurs. Another measure consists in applying a "black matrix" to the phosphor layer. The parts of the phosphor layer not used (spaces between the dots, landing reserve), as seen from the front, are covered by a black layer. In dark rooms, for example, today's cathode ray tubes supply contrast ratios of more than 1:200. In practical use however, the values are 1:40; see for example the book "Fernsehtechnik" [Television Technology," Huttig-Verlag, Heidelberg, 1988.
By contrast, light valve projectors in dark rooms offer a contrast of 1:60 and in bright rooms, due to the scattering of the projection wall, only 1:6. In the large screens that are in wide use today, in which a plurality of cathode ray tubes arranged side by side in rows are projected onto a large rear projection screen, however, the contrast gain of the "black matrix" screen is lost once more.
By comparison to other projection methods, laser projectors have a very high primary contrast of 1:300. Therefore, in semi-dark rooms, this permits a much better reproduction than with conventional projectors. In very bright rooms, or in daylight, however, this inherent advantage no longer applies since the contrast in this case is also determined primarily by the scattering properties and the basic brightness of the screen.
The goal of the invention is to provide a rear projection screen, preferably for laser projectors, which diffusely forward-scatters the narrow-band laser light in one or more colors with high efficiency at a specific solid angle, but allows the broad-band ambient light from the viewing area to pass largely unimpeded and be fed to a light absorber.
The purpose of this projection screen according to the invention is to be able to cause large images, even with normal ambient illumination in bright rooms or in daylight, to stand out from the bright environment. Secondly, the contrast originally available in the device is also to be reproduced on the screen for the viewer. Thirdly, distortion of color by ambient light is to be minimized.
This goal is achieved according to the invention by a holographic screen. The object for recording this holographic screen is preferably an adapted screen that is preferably illuminated with all of the laser projection wavelengths employed in the hologram. During recording, care is taken to make sure that the screen is illuminated with the object beam so that its forward scattering characteristics remain the same as will be desired later during application. As the reference beam in the holographic recording, an expanded beam bundle is used that originates from a suitable location like the later projection beam. For reproduction in the case of a hologram, either an extensive beam as in recording or a point- or linewise-scanning beam can be used.
The hologram is preferably recorded as a so-called off-axis transmission hologram, in other words it is suited for viewing in transmitted light. Both "thin" and "thick" holograms can be used as screen holograms. The decision as to which type to use reflects the available recording materials, their cost, the degree of refraction desired, and the type of reproduction. With thick holograms, a high angular and wavelength selectivity can be achieved in particular during reproduction.
In the recording proposed here of a thin hologram in one step, the screen is illuminated as a two-dimensional lattice structure. The known recording geometry according to Leith and Upatniek is used, with a divergent reference beam. During reproduction with a projection beam that corresponds to the reference beam, the screen appears as a virtual image (in first-order diffraction) and can also be used directly as such. The stray light components in zero-th order diffraction and the other orders of diffraction are minimized and absorbed outside the hologram. As in other holograms of this kind, the image of the screen appears behind the hologram plate at the same location as during recording. Such holographic screens are suitable for many applications, for example for PC screens, in order to increase virtually the reading distance to the screen and thus facilitate accommodation of the eyes.
It is proposed for other applications to make the transmission hologram of a screen in two stages. The first step is the same as that described above. In this case, however, instead of the virtual image, the real image of the screen is used as the object for a second recording, and then optimized. This has the advantage that the position of the screen relative to the hologram plate during reproduction can be freely selected. For many applications, for example for video, television, or large projection, it is logical for this screen to be in the plane of the hologram. With this recording technique, it is also possible to allow the screen to project in front of the hologram, which is especially attractive for advertising purposes or for artistic image design.
A white holographic screen, as mentioned above, can be produced advantageously by recording a screen with all of the laser wavelengths used, for example red, green, and blue (RGB) in the same hologram. There are three different design possibilities in this case. In the first, three exposures of all the colors can be made in one recording layer. In the second, several layers of different spectral sensitivities, to which various laser wavelengths are adapted, can be stacked. In the third, the various recording materials can be arranged side by side, for example pointwise as an RGB triplet within each image point in a triangular arrangement, like the phosphors of a television delta shadow mask tube or as three adjacent vertical RGB strips, like the phosphors in the familiar television Trinitron tubes.
Recording holograms to be assigned to three different colors in a single thin layer, as proposed in the first alternative method, has the problem that each individual lattice structure also diffracts light of other wavelengths. In such a screen, with three-color projection, nine different scattering lobes in four different colors result, three of which coincide in a white lobe which then supplies the actual viewing light. The the other scattering lobes may be suppressed by measures provided later, such as lamellar structures.
In a stacked arrangement of the layers, for example, three different recording materials adapted to the colors can be used for example. By using three laterally arranged layers of various colors, the color crosstalk, as in the cathode ray tubes of a color television set, can also be suppressed.
The invention also provides that "thick" transmission holograms are used for screen recordings, primarily in applications in which a high selectivity of the hologram relative to the reproduction wavelength and irradiation direction is advantageous.
In thick holograms, during recording, a volume lattice is formed in the recording layer that is 5 to 30 microns thick as a rule. During reproduction, then, because of the interference between adjacent partial beams that are phase-shifted with respect to one another, the Bragg condition holds for the design interference. Consequently, a high diffraction efficiency for the recording wavelengths and the illumination direction of the reference beam is integrated into the screen and broad-band diffuse light as well as laser light passes through from the viewing side largely unimpeded; see for example the book "Holographie" by J. Eichler and G. Ackermann, Springer-Verlag, 1993.
As in a "thin" hologram, either a plurality of recordings of the screen with different colors can be recorded in the same hologram layer, in different layers arranged side by side or in rows, with adapted color sensitivity. The special advantages of the holographic screen with both thin and thick recording materials is its property to produce only an image of the originally diffusely scattering object screen in rear projection with the recording wavelengths for the viewer, but to allow the diffuse broad-band light from the viewing area to pass through almost unimpeded, where it is subsequently absorbed. The holograms can preferably be on a glass disk which preferably is provided with a coating for visible light.
To suppress the stray light of the zero-th and first orders, the invention provides additional holograms or diaphragm structures.
By incorporating additional optical elements into the beam path of the reference beam or of the object beam, the holographic image of the screen can be influenced. Thus, for example, the radiation angle of the holographic screen can be modified in elevation and azimuth with respect to the original screen, the brightness distribution over the screen can be set differently, and image errors in the projection optics can be improved later.
The manufacture of such a rear projector and holographic screen according to the invention is described above as an example. However, it can be accomplished in a number of different ways and in different steps that are known by and understood by the individual skilled in the art. The invention is described below in greater detail with reference to the embodiments shown partially schematically in the figures.